This invention relates to a process of preparing water-dispersible granules and to water-dispersible granules produced by said process. The water-dispersible granules of the invention are formed by aggregating small microcapsules, which contain one or more water-insoluble pesticides within a polymeric shell wall, into larger spherical granules.
For some low melting solids/liquids, micro-encapsulation offers the only means of maintaining stable sprayable suspensions of that chemical in water. Having accomplished a stable suspension, microencapsulated pesticide formulations which are aqueous suspensions of microcapsules offer many desirable features. In some cases, reduced toxicity and extended activity of the encapsulated pesticide has been noted. Many pesticides decompose or volatilize quickly, thus reducing the effectiveness of the material; microencapsulation of such materials can postpone the decomposition of the pesticide. Micro-encapsulation of pesticidal materials can also enhance the safety of the pesticide for the applicator. Since the pesticide is enclosed in a polymeric shell wall the applicator is not directly exposed to the chemical. Another advantage of encapsulation of an active pesticidal agent lies in the possibility of a combination of substances that cannot be blended or are incompatible with one another, e.g., water-insoluble pesticides with water-soluble pesticides or water soluble fertilizers.
For liquid products, microencapsulation can eliminate the cost, availability, flamability, toxicology disadvantages of solvents by permitting water to be used as the suspending medium. For solid formulations derived from solutions or suspensions, those advantages generally pertain to the process as well as to the end use of the product.
Various methods are known in the art for microencapsulation of water-insoluble pesticides via interfacial polymerization reaction. U.S. Pat. Nos. 4,360,376, 3,429,827, 3,577,515 and 4,280,833 provide a good summary of the methods which are available. U.S. Pat. No. 4,280,833 describes the microencapsulation of concentrated amounts of water-insoluble pesticide materials on the order of 480-700 grams per liter, this previously unattainable high concentration offers a distinct energy saving advantage when water driveoff is required to convert the liquid to a solid.
While it can be advantageous to encapsulate pesticidal materials, it is also advantageous to have the pesticidal material in dry form. Dry pesticide formulations can be stored for long periods of time, over wide extremes of temperature, without destroying the stability of the formulation. It is easier and less expensive to dispose of containers in which dry pesticidal materials are stored since these are most often made of paper, which can be safely burned if the solid empties completely from the bag leaving no residue. Still more preferable is the use of water-soluble plastic bags in which to store, ship and add to water the water-dispersible granules or other dry-form pesticides. Shipping costs can be reduced since the solvent or water carrier of emulsifiable concentrate and water-based flowable pesticide formulations is eliminated. The most common types of dry pesticide formulations are wettable powder and granule formulations. The preparation of each type of formulation is known.
Various methods are described in the technical end patent literature for producing various forms of encapsulated products for pesticidal materials. Typically, the encapsulated product may be in the form of microcapsules of encapsulated material suspended in a carrier (continuous) medium or in the form of dried microcapsules, powders, grannulates, microgrannulates or water-dispersible granules. Examples of the foregoing products may be found in British Patent No. 2,042,892 and in the following U.S. Pat. Nos. 3,429,827, 4,244,836, 4,309, 213, 3,737,337, 4,157,983, 4,235,872, and 3,577,515.
Other forms of water-dispersible, non-encapsulated products are described in U.S. Pat. Nos. 3,657,446, 3,639,617, 3,954,439, 2,870,059, 3,920,442, 4,511,395, 4,134,725, 4,183,740 and 3,854,981, in German Patent No. DT-1,642,122 and So. African Application No. 692053. The products produced in the above exemplified patents may be dried typically in an air flow, by evaporation, by spray drying, etc.
Yet another common form of pesticidal product mentioned above is the wettable powder, as exemplified in British Patent No. 2,037,585 and U.S. Pat. Nos. 3,791,811 and 3,731,551.
The most common wettable powder pesticide formulation is one in which fine particles of a high melting solid, crystaline pesticide are combined with a finely-divided solid carrier, e.g., silicates or alumino silicates comprising single lattice or double lattice clays. The surface active agents allow the concentrate to be diluted in water to field strength to form stable, sprayable suspensions. Most high melting solid pesticides can be processed as a wettable powder in the 60-90% concentration range; the main requirement being that they exhibit appropriate chemical compatibility with the finely divided, solid carrier. In order for low melting actives to be formulated as wettable powders they must first be absorbed into highly porous media such as diatomaeous earth, pearlite or manufactured silicas to a degree that the mixture exhibits the properties of a solid. This dilution essentially reduces wettable powder concentrations of liquids to less than 60%, with 40-50% a more common range. Further, low melting actives which undergo a phase change at ambient storage temperatures (e.g., -1020 to +50.degree. C.) usually present such a caking problem from crystal formation and growth that the use of an absorbing agent is futile and cake-free solid formulations having concentrations of &gt;20% are impossible to attain. Alachlor and trillate are excellent examples of such chemicals.
The primary disadvantage of wettable powder pesticide formulations is that they tend to be dusty, posing health problems to the applicator if the pesticide material is irritating or toxic and handling problems during the preparation of the material. Further, wettable powders tend to have low bulk density and, thus, do not wet up rapidly when added to water. Inability to wet up can result in excessive mixing times and/or the formation of "lumps" of wettable powder in the water. These lumps are difficult to disperse in the water making it difficult to get even distribution of the pesticide throughout the water and, thus, even application of the pesticide when applied. In extreme cases, lumping can result in clogging of sprayer nozzles.
A granular pesticide formulation is one which usually involves impregnation of molten pesticidal agent into the pores of a preformed granule, but may involve agglomeration of high melting solids with powdered inerts such as clay. For impregnation, the active ingredient must be a liquid at a temperature below about 120.degree. C. or be compatible with a liquid carrier which can take solubilized or suspended active ingredients into the pores of the granule during a liquid-solid blending operation. Common granule carriers are clays, attapulgite, bentonite, sepiolite and the like.
Granular pesticides comprising high-melting solids may also be formed by extrusion, agglomeration or core coating.
A disadvantage of some conventional granule pesticide formulations is that the total amount of active pesticide carried on the granule is limited by carrier, equipment metering and efficacy considerations to 5-25% loadings. These low loadings contribute to the expense of the granules. This, and the added expense of separate granular application equipment, are economic limitations on granule use. Caking can be a problem if the active pesticidal agent readily migrates to the surface of the granule making it "sticky". Finally, since many granules are irregularly shaped, some dusting occurs as the granules wear against each other in the package during storage and handling.
Although, as indicated above, water-dispersible granules are generally known and have been commercially available for sometime now, the present invention affords an economical means of making water-dispersible granules from chemicals and combinations thereof having a wide range of melting points, including low melters. The process according to this invention permits the formation of granules having a particularly advantageous structure and physical properties and formulation compatible with that process.
As will be described in more detail herein, an important portion of the water-dispersible granule of the invention is an aggregate of many small polymeric microcapsules containing a water-insoluble pesticide. The geometry and composition of the water-dispersible granules of the invention permit them to be free flowing and relatively dust-free. Since the active pesticidal agent is encapsulated, the water-dispersible granules of the invention pose very little hazard to the user when handled. Another advantage of microencapsulated water-dispersible granules is the ability to produce a product package containing a plurality of pesticides wherein antagonistic action between the pesticides is reduced or eliminated by means of the capsule shell.
The water-dispersible granules of the invention have bulk densities sufficiently high to readily wet-up when added to the water in a farmer's spray tank thus eliminating the wetting or lumping problem of wettable powders.
The water-dispersible granules of the invention immediately reconstitute when added to water; by that it is meant that the large aggregates dissociate or break apart into the tiny, individual microcapsules which disperse to their original pre-agglomerated form throughout the water. Since the pesticide is encapsulated, one can get a high degree of loading of the active, on the order of 65-90% active pesticidal agent for water-dispersible granule as contrasted to the maximum 50% loading attainable with commercially available granules. Further, one is able to add the encapsulated pesticide to aqueous solutions, e.g., liquid fertilizer solutions which might ordinarily be antagonistic to the unencapsulated pesticide.